I was reading Desert Solitare by Edward Abbey, and in it, he mentions that he was using bull snakes (bull snakes are non-venomous) during his time as a park ranger to help control the mouse population around his trailer and to keep rattlesnakes away.
While I am certain that bull snakes would help keep the mice under control, is there any evidence that they keep rattlesnakes away?
Here is a quote from Wikipedia:
The idea that bullsnakes occasionally eat rattlesnakes is sometimes given as a reason for humans not to harm bullsnakes when encountering them in the wild, although a better reason is the bullsnake's role in controlling warm-blooded vermin such as rodents.
While this may occasionally happen, the truth is bull snakes don't really affect rattle snake population apart from competition for the same food:
A thorough search of the literature and discussions with researchers who study both snakes has revealed next to nothing that supports the idea that bullsnakes eat rattlesnakes. Bullsnakes are primarily consumers of warm-blooded prey. In one instance, the body of a small rattlesnake showed up in the gut of a bullsnake, but no information exists on whether the ingested rattlesnake was already deceased or even what species it was. It is possible that a young bullsnake may eat a lizard, but no rattlesnake population could be significantly affected by bullsnakes. The natural mortal enemy of rattlesnakes is, in fact, the kingsnake. Additional information, Nov. 27, 2013: An article I ran across says that it happens, but rarely and largely opportunistically. In the particular study, looking at the guts contents of over 1000 bullsnakes, 2 rattlesnakes were found. This is 0.5% of the entire list of prey items found. We don’t know if the rattlers were constricted alive and eaten or scavenged, unfortunately. The take home lesson is still that even eating one or two rattlers during its lifetime, for a bullsnake, this consumption rate is too small to exact any population control over rattlesnakes.
So the answer to your question is essentially no they don't really keep rattle snakes away.
Interestingly, Bull snakes developed an immunity to Rattlesnake venom. Evolutionary pressures would be the most likely reason for that immunity. Where did that pressure come from?
This article in the Smithsonian Magazine notes that some animals, including snakes, are immune to some snake venoms, and seems to suggest that the eating of the venomous snakes by the snakes that developed immunity to the venom of the eaten snake may be the cause.
Besides mammals and lizards, there are plenty of snakes that are immune to snake venom. In some cases, it may be that immunity prevents the serpents from inadvertently committing suicide when they miss a mouse and hit themselves instead. (You know what it’s like to bite your cheek while you’re eating? Now imagine you’re venomous.) But in other cases, immunity points towards ophiophagy, or snake-eating.
This cause is not clear, because this Wikipedia article suggests otherwise.
Many ophiophagous animals seem to be immune to the venom of the usual snakes they prey and feed upon. The phenomenon was studied in the mussurana by the Brazilian scientist Vital Brazil. They have antihemorrhagic and antineurotoxic antibodies in their blood. The Virginia opossum (Didelphis virginiana) has been found to have the most resistance towards snake venom. This immunity is not acquired and has probably evolved as an adaptation to predation by venomous snakes in their habitat.[4]
Bull Snakes are thought by some to eat rattlesnakes, but the accepted answer, above, found only very weak evidence of this. Possibly their mere presence in the environment can be sensed by Rattlers and likely creates evasion behavior. Certainly, the immunity had to be created by very close contact with Rattlers which in turn gives evolutionary pressure.
